This invention generally relates to casks for transporting nuclear fuel rods, and is specifically concerned with a modular cell wall that provides an improved basket structure for use within such a transportation cask.
Basket structures for use in conjunction with fuel rod transportation casks are known in the prior art. Generally, such baskets are formed from an "eggcrate" of stainless steel plates that are slotted and interfitted to define an array of square, elongated cells. Each of the cells holds an elongated fuel rod container that has a square cross-section. The cells of the eggcrate space the fuel rod containers from one another so as to maintain a subcritical state when the cells are completely loaded with nuclear fuel. As further assurance from subcriticality, the sides of each of the fuel rod containers are typically clad with a neutron-absorbing material, such as Boral.RTM.. The eggcrate of the basket structure is insertable into and removable from a cylindrical transportation casks that is typically formed from carbon steel which may be up to one foot thick. During the loading operation, the entire transportation cask is hoisted down into and completely submerged within a waterfilled cask-loading shaft on the premises of the nuclear reactor facility. Spent nuclear fuel rods are loaded into the containers held within the cells of the eggcrate of the basket structure. These spent fuel rods may either be held together in spent fuel assemblies or in consolidation canisters, either of which is receivable within the fuel rod container held in the cells of the eggcrate. After the loading operation has been completed, the entire transportation cask is hoisted upwardly by a crane above the surface of the water in the shaft, and allowed to drain. Thereafter, the cask is then lidded, and the fuel rods are transported to their final destination.
Unfortunately, such prior art basket structures are not without shortcomings. One of the most serious of these shortcomings is the tendency of the stainless steel plates in eggcrate type designs to warp in response to thermal and mechanical stress, which may result in the mis-shaping of one or more of the containers in the basket cells. Even relatively small amounts of warping may render the individual container useless, as the inner dimensions of the container are designed to hold either a spent fuel assembly or a consolidated fuel container with an absolute minimum amount of lateral "slack" so that the fuel rod assembly or fuel rod container will not tend to rattle within the basket structure when the transportation cask is in motion. Still another shortcoming associated with eggcrate type basket designs is their relative difficulty of manufacture. The interfitting slots in the plates that form the basket must be cut with a great deal of precision if the resulting cells are to be highly uniform in size. Moreover, when two interfitting plates are welded together along their mutually interlocking slots, the resulting weld beads can contract the metal along the slots to such an extent that the square cross sections of the eggcrate cells become warped into rhombi. If one attempts to correct such weld warpage by the application of counterwelds, one sometimes straightens one row of cells at the expense of buckling another row. It has been suggested that the warpage problem might be solved by merely increasing the size of the cells. Such a solution, however, would significantly increase the size and weight of the basket structure and would not necessarily solve the problem. Moreover, the greater amount of slack space resulting from such larger cells would create other problems that would have to be dealt with. A final shortcoming of eggcrate type designs is the difficulty of removing the rods from the basket in the event the cask is exposed to a damaging amount of mechanical shock. The mechanical interdependency of the interlocking plates may cause the entire basket structure to undergo severe warpage in a case where the casks is dropped on its side with one set of corners of the eggcrate vertically aligned, which in turn would tend to misshape all of the cells into flattened rhombi. Such a flattened rhombus shape would in turn cause the walls of the cells to pinch the fuel rods contained therein, thereby impeding removal.
Clearly, what is needed is an improved basket structure for use in a fuel rod transportation cask that is relatively simple and inexpensive to manufacture tosamll tolerances, but yet which will maintain such dimensions tolerances throughout a broad range of thermal and mechanical stress. Ideally, such a structure would be devoid of small crevices and recesses so that the cask is easily drainable, and have good heat transfer characteristics so that residual heat from the spent fuel rods will be readily dissipated into the ambient atmosphere. Finally, the cells in the basket structure should be easily repairable in the case of an accident.